1. Technical Field
The present invention relates to self-elevating marine vessels known as "jack-up barges," "jack-up rigs or platforms" or "-up units," as typically used in the offshore oil industry, usually as mobile oil well drilling rigs. More particularly, the present invention relates to a system for inclining the movable, telescoping independent legs of a jack-up unit with respect to the hull portion in order to improve stability in bad weather conditions, for example. The system spreads the feet (cans) of the legs and at the same time maintains fixation between all chords of the legs and the unit's platform or hull. Spreading the feet reduces the leg load due to overturning moments, caused by winds, waves, currents, dynamic motions, and "punch thru." The fixation system lessens the introduction of leg chord stresses due to horizontal guide forces for absorbing leg moments by transmitting the chords' axial stresses directly into the jack tower.
2. General Background
The terms "jack-up rig," "jack-up barge" and "jack-up unit" relate to mobile marine vessels having three or more independent legs, each movably affixed to a floating or floatable barge-like hull or platform. The legs can change elevation with respect to the hull so that the legs can extend to the sea bottom and form a structural base for the hull. The barge-like hull can then be elevated using powered jacks contained within structural jack towers mounted on the hull. The jacking mechanism is used to effect changes in vertical elevation between the hull or platform and the legs. Many jack-up units use a rack and pinion-type jacking system to elevate the hull with respect to the legs. An early article entitled "Platform Jacking Mechanisms," showing a jack-up unit with an opposed pinion, rack and pinion-type contin"o"s jacking mechanism was published in Lapic Industrial Opportunities by Kenneth Mason Publications, Ltd., 107 Victoria Street, London 8WI.UK., Issue No. 37, October 1961. Various patents also describe rack and pinion-type jacking mechanisms for jack-up units. Such a system is shown, for example, in FIGS. 6 and 11 of R. G. LeToureau's U.S. Pat. No. 3,183,676 entitled "Mobile Sea Platform," issued on May 18, 1965. The Wilke et al. patent entitled "Leg Supported Offshore Structure with Jacking Apparatus" shows another such rack and pinion type jackup barge. That patent was originally issued as U.S. Pat. No. 3,606,251 on Sept. 20, 1971, and then reissued as U.S. Pat. Re. 29,539 on Feb. 14, 1978 and assigned to Armco Steel Corp. U.S. Pat. No. 4,269,545 issued on May 26, 1981 to J. L. Goldman et al. shows a fixation system employing a "rack chock" for rigidly attaching the legs to the hull of a jack-up unit.
In 1964 to 1966, Marathon Le Tourneau built mobile offshore jack-up drilling units with tilting legs. These units which included Zapata Off-Shore Company units named "Chaparral," "Endeavour," "Heron," and "Intrepid," were three chord legs, three legs per unit and had only the inboard chords of each legs attached to the hull by elevating jack and guides. In 1967 and 1972, Marathon Le Tourneau built similar units with tilting legs including Zapata Off-shore Company units named "Explorer" and "Nordic." These units have three legs with four chords per leg. The legs are carried by a structure which is the full depth of the hulls with the elevating jacks being part of the structure and located above the main deck. This supporting structure pivoted about two points which are on the vertical center line of the legs, opposite sides of the leg and half of the hull depth. These support methods were designed to take the leg moments due to storm loadings as a horizontal couple, which creates a higher chord stress. These are shown in the September, 1983 issue of "Ocean Industry" as part of the 1983-84 Directory of Marine Drilling Rigs. (Note for example page 176 of the September 1983 issue of Ocean Industry.) These units did not use a positioning frame (jack tower) to change the inclination of the legs, yet provide rigidity between the legs and the hull and transmit the leg loads directly into the hull. A similar system with inwardly sloping legs is in the Durand, et al. U.S. Pat. No. 4,437,792. The legs of Durand et al. can be interconnected at the center of the platform to form a pyramid for the purpose of eliminating stresses on the legs during towing.
In order to insure adequate foundations for the unit's footing on the sea bottom, additional sea ballast is usually added prior to elevating the unit, so that the total bearing pressure is greater than that expected during the maximum storm conditions, when the unit is elevated. This additional load, called "preload," pushes the legs down into the sea bottom when it is soft. There often exists sand strata in the seabed, which are hard layers that must be penetrated. Penetrating these sand strata layers has frequently caused problems. The procedure can be difficult and damaging to the legs. The damage occurs, for example, when there is rapid settling of one leg only. The unit leans over, placing the legs in bending.
Existing truss-type independent leg jack-up units are three or four chord leg designs. Chords have been fabricated from inter alia, straight or curved plates, pipe sections, or rolled structural members. Chord size and shape determines the wave loadings. Smaller chords are subject to higher secondary bending stresses in the guides for equal bay lengths. The chords slide upon the guiding surfaces in the jack towers or upon the hull. Since it is not possible to provide adequate lubrication with extremely high loads, wear on the rack teeth and/or chord surfaces and guides can be extensive and damaging.
Existing jack-up rigs utilize a fixed jack tower. Such affixed jack towers can be seen, for example, in FIGS. 1B, 9B and 2 of the Goldman et al U.S. Pat. No. 4,269,543, FIG. 1A of the Choate U.S. Pat. No. 4,422,802, FIG. 1 of the Lovie U.S. Pat. No. 3,967,457, and FIGS. 2 and 9 of the Wilke reissue U.S. Pat. No. 29,539. The jack tower structures incorporate the elevating jack and guides.